An electromagnetically actuated piston slide valve can be used as a throttle valve in a hydraulic shock absorber of a vehicle in order to adjust a shock-absorber characteristic to be “hard” or “soft”. By means of the adjustable throttle valve the flow resistance of the valve and thereby the shock-absorbing effect of the entire system can be changed in dependence on the electrical energizing of the field coil of the valve. The valve connects two shock-absorber chambers here, wherein pressure surges on the shock absorber cause a fluid displacement from one shock-absorber chamber into the other shock-absorber chamber.
In dependence on the application, it may be required that the valve is closed (“normal closed”, NC) or open (“normal open”, NO) in the currentless state. When the valve takes a predetermined position in the currentless state, this is also referred to as fail-safe state, since the valve takes this state when the entire system is turned off or fails, for example when the power supply breaks down. This fail-safe function is used in shock-absorbers for motor vehicles, for example. It can be advantageous when the fail-safe state defines a partly opened state of the valve, so that in the case of a system failure the shock absorber does not switch to a very soft or hard setting, in order to ensure a moderate and secure driving condition thereby.
From DE 10 2008 035 899 A1 and DE 10 2013 106 214 A1 electromagnetically actuated NO valves are known. These valves have a fail-safe position in which the valve is partly opened, i.e. a position between the maximally opened and closed the position in the unenergized state. When the coil of the valve is electrically energized, the piston (also referred to as slide) of the valve initially moves to the maximally opened position and can be held there with a basic energy supply, i.e. a minimum energy supply that is required for keeping the valve maximally open. When the current is further increased, the slide moves continuously in the direction of the closed position.
These valves have two magnetic armatures and two corresponding biasing springs. In order to hold the valve in the maximally opened position, a minimal magnetic force is necessary and in a “basic energy supply” in order to overcome the force of the fail-safe spring, i.e. that spring which urges the piston in the direction of the fail-safe position. When the energy supply is lowered below the basic energy supply, the valve switches to the fail-safe state. A lowering of the basic energy supply is thus impossible. Also external influences, such as vibrations due to unevenness of the road surface, can likewise have the result that with basic energy supply the valve unintentionally switches from the maximally opened position into the fail-safe position. Moreover, the fail-safe spring cannot be configured with any desired stiffness, and thus the fail-safe stroke cannot be configured with any desired dimension, since the valve would otherwise switch to the fail-safe state too easily. Put differently, a stiff fail-safe spring requires a high basic energy supply for overcoming the biasing force of the fail-safe spring and for holding the maximally opened position of the valve.